1. Field
The method and system herein relates to telescopes, and more specifically to a wide field of view reflective telescope having a flat focal plane array.
2. Related Art
Classically, space situational awareness includes two rather separate functions: surveillance to detect orbiting objects, and tracking observations to confirm and characterize the object, and provide data from which its function and operational state can be inferred. At times it is desirable to use passive, all-optical techniques for space situational awareness. Tracking is relatively simple once a satellite's ephemeris is known. However, finding unknown objects requires a blind search, undirected by prior knowledge of orbital elements, size, compositions, or aspect. This is observationally intensive and slow. At present, optical tracking systems look for satellites in higher orbits where their angular rate relative to a ground observer is comparatively slow.
However, most satellites are located in lower altitudes. Approximately 80% of all satellites are located at or below 1250 km above the surface of the earth. Objects in low earth orbit (LEO) pose a significant challenge to a blind search as they move at high angular rates, in many cases—due to size or reflection characteristics—represent dim targets, and the effective field of view for telescopes at low altitudes is quite restricted.
There are optical systems that search for orbiting objects, though historically, for LEO objects this function has been largely fulfilled by radar systems. It would be prudent to use available technology to advance space situational awareness, including passive optical systems.
Micro-, pico-, and nano-satellites in low earth orbit pose an interesting challenge for passive optical detection and tracking. In general a micro-satellite would be expected to have linear dimensions somewhere between about 10 cm and 30 cm. Such objects have visual signatures in the range of 12th to 16th magnitude when viewed in terminator at a range of 500 km. While not extremely dim, micro-satellites represent a challenging target for smaller telescopes. Nonetheless, optical components of the Space Surveillance Network (SSN) Ground Based Electro-optical Deep Space Surveillance (GEODSS) telescope and other telescopes operated for deep space searching frequently detect and produce track information for even dimmer satellites, but they do so only for objects in high orbits. The most difficult problem posed by microsatellites is that of blind optical search for systems in LEO.
When working at LEO altitudes optical telescopes encounter two significant difficulties. While the satellites are close and therefore slightly brighter, by Kepler's Third Law they move more rapidly than at higher altitudes and the effective field of view of the telescope is decreased. While the first problem is simple physics, the second is one of relative angles and is not entirely intuitive.
A telescope field of view (FOV) is determined by its design parameters including the effective focal length, charge-coupled device (CCD) size, and impact of vignetting. While a 5° FOV telescope will see a patch of sky 5° across, it will not observe the equivalent angular extent of a LEO satellite orbit. According to simple geometry, arc length is the product of angle and distance, such as ArcLength=Range*θ. A ground-based telescope observing satellites at 500 km can see an arc of length 500*FOV km, but the angular sweep of a satellite moving through this same distance is much less than the telescope FOV.
Satellite motion is defined relative to the center of the earth. Hence, the angular sweep for the satellite crossing the telescope FOV is given as θorbit=500*FOV/(500+EarthRadius). As these arcs are short and LEO satellites move quickly, the transit time along this arc will be brief. This results in the angular velocity of a LEO satellite through a telescope FOV quite high. For example, while a geosynchronous satellite appears motionless, a LEO bird appears to move at more than 228 arc seconds per second, a rate more than 15 times greater than stars appear to move across the sky.
Of the many telescopes in operation world wide, few are dedicated to wide area search. Even within the U.S. Air Force, where several moderate aperture telescopes are available, most are narrow FOV instruments. The U.S. Air Force GEODSS program has the only telescopes dedicated to wide-area satellite search. Outside of the U.S. government, other capabilities exist but these are largely dedicated to astronomical sky surveys and the search for near earth objects (NEOs). These surveys all have some inherent capability to detect slow moving satellites but with few exceptions. Most surveys ignore satellites while some actually find them to be an impediment. All of these systems cold detect the streak of a bright LEO satellite that happens to pass through the FOV, but none are optimized for detecting the very brief streaks of dim LEO objects.
A need exists for a system of wide field of view telescopes for finding LEO satellites and space objects, which are often difficult to detect and track, as well as for astronomical research. Such a system could be used to generate initial track information sufficient for acquisition and fine tracking by more traditional slewing telescopes.